Discount price doesn't work

Two prices are shown on your page because you have two prices on your product. You should have only one.
Then, you should create a discount via the menu System->Discounts. That discount would then be applied on the price of the product and you would see the striked price and the discounted price on the product page based on the option of the configuration to show the discounted price.
If you want to apply a discount only to a category of products or only to a product, that is only possible in the commercial editions but you can already apply discounts to all the products with the Starter edition.

It's the option 'Show discounted price' of the Display tab of the config with the value "Display the price before the discount is applied". That will strike automatically the undiscounted price that you set in your product.